(CNN) -- Last-minute planners beware: Spirit Airlines has added fees for travelers who wait to pay for carry-on or checked luggage within 24 hours of departure.

Calling its previous bag fees an "Early Bird Discount," the airline will now charge travelers who pay for checked or carry-on bags within 24 hours of departure online an extra $5 and an extra $10 for travelers who pay over the phone.

"With this updated policy, fees remain the same for customers who pre-purchase their bags at least 24 hours in advance via spirit.com or through reservations," said spokeswoman Misty Pinson via e-mail.

"We are continuing to pass savings along to our customers who choose self-service options vs. waiting until they get to the aiport to conduct their transactions."

Fliers who pay for carry-ons early -- and aren't members of the airline's fare club -- pay $30 for a carry-on that won't fit under the seat. Spirit became the only U.S. carrier to charge for carry-on bags when it instituted new fees in April 2010.

Paying for carry-on bags at the airport now will cost $40 at the airport check-in counter or kiosk and $45 at the gate.

The changes are in effect for travelers who booked on or after March 24.

Spirit Airlines also has dropped its threshold for overweight checked bags from 50 pounds to 40 pounds.

So if your bag weighs between 41 and 50 pounds, you'll pay an additional $25 on top of whatever you paid to check the bag. Checked bag fees start at $28 for the first bag checked online or by phone -- 24 hours in advance -- for travelers who aren't part of the airline's fare club. The new weight limits went into effect for travel after February 1, Pinson said.

Other U.S. airlines start charging fees for overweight luggage at 50 pounds.